1. Field of the Invention
The present invention relates to an apparatus for forming an image on a recording member.
2. Related Background Art
There is already known an apparatus for forming an image, through an electrophotographic process, by charging and exposing a photosensitive member to form an electrostatic latent image thereon and developing said latent image.
The sensitivity of a photosensitive member varies with the lapse of time or by a change in the environmental conditions such as temperature and humidity. In an apparatus for forming a dot-pattern image on a photosensitive member by means of a laser beam or the like, there is often employed a method of modulating each pixel with a predetermined area rate. However, in such method, control has been accomplished with predetermined area rates for the start and stop of laser beam emission, since the points of said start and stop are inevitably distant in time. It is that known time-dependent changes occur in the potential V.sub.OO of the photosensitive member at the start of laser beam emission and the potential V.sub.FF at the end of laser beam emission, as shown in FIG. 13.
FIG. 13 shows the surface potential as a function of the grid voltage VG of the primary charger. Since the surface potential V.sub.OO varies from A to B in time while the surface potential V.sub.FF varies from C to D, it is necessary to vary the grid voltage from 700 V to 1000 V in order to attain a same value of V.sub.c =(V.sub.OO -V.sub.FF) =420 V. However it has been difficult to precisely cover a voltage range from 200 V to 1000 V in a high-voltage unit.
Also the performance of the developing unit varies with the ambient conditions. Particularly the image density is affected by the humidity, and it is often not possible to obtain an optimum density.